Ultra-fast tunable optical filters

ABSTRACT

An optical filter including at least one multiport optical coupler formed on a gallium arsenide substrate, one connection port of the at least one multiport optical coupler receiving an input output signal, and another connection port of the at least one multiport optical coupler outputting a filtered optical signal and at least one electrically tunable optical resonator, formed on the gallium arsenide substrate and connected to at least one of the at least multiport optical coupler.

This application is a continuation of application U.S. Ser. No.09/230,959 filed Apr. 8, 1999, now U.S. Pat. No. 6,222,964 which is a371 of International Application No. PCT/IL97/00264 filed Aug. 3, 1997,which claims priority to Israel Application No. 119006 filed Aug. 4,1996.

FIELD OF THE INVENTION

The present invention relates to the field of tunable optical filters,especially for use in optical communications systems.

BACKGROUND OF THE INVENTION

High-speed data communications systems need to support the aggregatebandwidth requirements of current and future applications such assupercomputer interconnection, high-quality video conferencing, andmultimedia traffic. It has long been clear that these bandwidthrequirements can only be met by using optical transmission technologies.Many current approaches favor packet switching and ATM (asynchronoustransfer mode) technology, due to their flexibility. The most promisingcandidate for the future hardware backbone for such networks is denseoptical WDM (wavelength division multiplexing), a method of multiplexinga large number of optical data channels on a wavelength basis, i.e. eachwavelength is regarded as a different channel, and is routed andmanipulated separately from all other wavelengths.

Dense WDM needs advanced optoelectronic components and subsystems,capable of handling the extremely high aggregate bit rates and trafficlevels demanded by modern optical data communications systems. One ofthe most critical components needed for implementation of WDMpacket-switched systems is an ultra-fast tunable filter—a wavelengthselective element in which the central wavelength of the selectedbandpass can be tuned externally and dynamically at a very high rate.

Fast tunable filters are known and available commercially, but thetuning speed of all current known types falls far short of therequirements of future and even of some current optical datatransmission systems. The most common optical filters are based onclassical interferometers, and include Fabry-Perot and Bragg filters.Such filters are tuned by mechanically moving the resonator structure,and the tuning speed is therefore comparatively slow—typically of theorder of milliseconds, or, for the very fastest types, several tons ofmicroseconds.

Another type of tunable filter is based on the Acousto-Optical effect.Such components depend on the interaction between an acoustic wavegenerated in the device, and the optical signal inputted to thecomponent. Tunability is achieved by altering the frequency of theacoustic wave, which can be simply accomplished by altering thefrequency of the electronic signal used to generate the acoustic wave.These filters are, however, polarization dependent, which causes manypractical problems. Tuning speeds are reasonable high, of the order ofmicroseconds.

Yet another tunable filter is based on a micromachined semiconductorstructure, where the thickness of one of the parts of the structure isaltered electrically. Here too, tuning speeds of the order ofmicroseconds can be achieved.

The next generation packet-switched WDM networks are being designed foruse with traffic throughputs of the order of Tbits/sec. Such systemstherefore require switching and tuning speeds of the order of onenanosecond, and it is evident that even the fastest of the abovementioned filter technologies falls woefully short of theserequirements, by about three orders of magnitude.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved high speed tunableoptical filter which overcomes disadvantages and drawbacks of existingtunable optical filters, which provides tuning speeds of the order ofone nanosecond, and which is capable of implementation is a low cost,high production volume monolithic component.

There is thus provided in accordance with a preferred embodiment of theinvention, a tunable optical filter including at least one multiportoptical coupler, to one connection port of which is inputted an opticalsignal, and from another one of which is outputted an optical signal tothe end user, and optical transmission line of predetermined lengthconfigured as a resonator, with one of its ends connected to yet anotherone of the connection ports, and its other end connected to stillanother one of the connection ports, and with a phase modulator insertedin the above mentioned resonator such that the interaction of the phasemodulated signal in the resonator with the input signal allows onlysignals of a preselected wavelength to be transmitted from the outputport to the end user.

In accordance with another preferred embodiment of the invention, theoutput signal can be extracted from the resonator, by means of anadditional coupler inserted into the resonator. In this case, the use ofthree-port couplers is sufficient.

In accordance with a further preferred embodiment of the invention, afilter with variable finesse can be provided by the use of couplers withvariable power splitting ratios.

In accordance with yet another preferred embodiment of the invention,there is provided a compound resonator tunable optical filter includingat least three multiport optical couplers, to one of the ports of thefirst coupler is inputted an optical signal, and from one of the portsof the last coupler is outputted a filtered optical signal to the enduser, at least two optical resonators of predetermined lengthinterconnecting the free ports of the couplers, and at least one phasemodulator inserted in at least one of the above mentioned opticalresonators, such that the interaction of the phase modulated signal withthe input signal to the first coupler is operative to allow only signalsof a preselected wavelength to be transmitted from the output port ofthe last coupler to the end user.

In applications where an electronic signal is required for the end use,the optical output signal of the filter may be converted into such asignal by means of a fast photodetector mounted on the output port. Inapplications where an optical signal is required for the end use, suchas for optical spectrum analysis, the output optical data signal may beutilized directly.

In accordance with a further preferred embodiment of the invention,there is provided an optical filter including at least one multiportoptical coupler, one connection port of the at least one multiportoptical coupler receiving an input optical signal, and anotherconnection port of the at least one multiport optical coupler outputtinga filtered optical signal, and at least one tunable optical resonatorconnected to at least one of the at least one multiport optical coupler.

In accordance with yet another preferred embodiment of the invention,there is provided an optical filter as described above and wherein theat least one multiport optical coupler has at least first, second, thirdand fourth connection ports, the first connection port receiving anoptical signal, the second connection port outputting a filtered opticalsignal and the at least one tunable optical resonator being connectedacross the third and fourth connection ports.

In accordance with still another preferred embodiment of the invention,there is provided an optical filter as described above and wherein theat least one multiport optical coupler consists of at least first andsecond optical couplers, each having at least first, second and thirdconnection ports, the first connection port of the first optical couplerreceives an optical signal, the second and third connection ports of thefirst optical coupler are coupled to the at least one tunable opticalresonator, the first and second connection ports of the second opticalcoupler are coupled to the at least one tunable optical resonator, andthe third connection port of the second optical coupler outputs afiltered optical signal.

In accordance with another preferred embodiment of the invention, thereis provided an optical filter as described above and wherein the atleast one tunable optical resonator is operative to select an opticalsignal with a specific wavelength or to enable the polarization of thefiltered optical output signal to be selected.

In accordance with yet a further preferred embodiment of the invention,there is provided an optical filter with variable finesse consisting ofan optical element with variable finesse receiving an optical signal andproviding a filtered output, and a finesse controller operative toselect the finesse of the variable finesse optical element. The variablefinesse optical element of this embodiment could consist of an opticalcoupler with variable power splitting ratio between its connectionports.

In accordance with still another preferred embodiment of the invention,there is provided an optical filter with variable finesse consisting ofat least one multiport optical coupler with variable power splittingratio, one connection port of the at least one multiport optical couplerreceiving an input optical signal, and another connection port of the atleast one multiport optical coupler outputting a filtered opticalsignal, and at least one tunable optical resonator connected to at leastone of the at least one multiport optical couplers.

There is provided in accordance with a further preferred embodiment ofthe invention, an optical filter with variable finesse wherein the atleast one multiport optical coupler with variable power splitting ratio,has at least first, second, third and fourth connection ports, the firstconnection port receiving an optical signal, and the second connectionport outputting a filtered optical signal, and the at least one tunableoptical resonator being connected across the third and fourth connectionports.

In accordance with still another preferred embodiment of the invention,there is provided an optical filter with variable finesse wherein the atleast one multiport optical coupler with variable power splitting ratioconsists of at least first and second optical couplers, at least one ofwhich has variable power splitting ratio, and each having at leastfirst, second and third connection ports, the first connection port ofthe first optical coupler receives an optical signal, the second andthird connection ports of the first optical coupler are coupled to theat least one tunable optical resonator, the first and second connectionports of the second optical coupler are coupled to the at least onetunable optical resonator, and the third connection port of the secondoptical coupler outputs a filtered optical signal.

In addition, there is provided in accordance with another preferredembodiment of the invention, an optical filter with variable finessewherein the at least one tunable optical resonator is operative toselect an optical signal with a specific wavelength, thereby providingtunability to both the wavelength and finesse of the optical filter, orenabling the polarization of the filtered optical output signal to beselected.

Additionally, there is provided in accordance with still anotherpreferred embodiment of the invention, an integrated optical filterconsisting of at least one multiport optical coupler, one connectionport of the at least one multiport optical coupler receiving an inputoptical signal, and another connection port of the at least onemultiport optical coupler outputting a filtered optical signal, and atleast one optical resonator connected to at least one of the at leastone multiport optical coupler, and wherein at least one of the at leastone multiport optical coupler and the at least one optical resonator areformed on an integrated optical substrate.

In accordance with yet another preferred embodiment of the invention,there is provided an integrated optical filter wherein the at least onemultiport optical coupler has at least first, second, third and fourthconnection ports, the first connection port receiving an optical signal,the second connection port outputting a filtered optical signal, and theat least one optical resonator is connected across the third and fourthconnection ports, and wherein at least one of the at least one multiportoptical coupler and the at least one optical resonator are formed on anintegrated optical substrate.

Additionally, there is provided in accordance with a further preferredembodiment of the invention, an integrated optical filter wherein the atleast one multiport optical coupler consists of at least first andsecond optical couplers, each having at least first, second and thirdconnection ports, the first connection port of the first optical couplerreceives an optical signal, the second and third connection ports of thefirst optical coupler are coupled to the at least one optical resonator,the first and second connection ports of the second optical coupler arecoupled to the at least one optical resonator, and the third connectionport of the second optical coupler outputs a filtered optical signal,and wherein at least one of the at least one multiport optical couplerand the at least one optical resonator are formed on an integratedoptical substrate.

In addition, there is provided in accordance with other preferredembodiments of the invention, an integrated optical filter wherein theat least one optical resonator is a tunable optical resonator, or inwhich at least one of the at least one optical couplers and the at leastone optical resonators includes a discrete non-integrated opticalcomponent.

In accordance with still another preferred embodiment of the invention,there is provided an optical filter of the compound resonator type,consisting of at least three optical couplers, each having at leastthree connection ports, and at least two optical resonators, at leastone of which is tunable, each of the at least two optical resonatorsbeing connected between two of the at least three optical couplers, andwherein the first connection port of the first optical coupler receivesan input optical signal, and the last connection port of the lastoptical coupler outputs a filtered optical signal.

Additionally, there is provided in accordance with a further preferredembodiment of the invention, an optical filter of the compound resonatortype, consisting of at least first, second and third optical couplerseach having at least first, second and third connection ports, and atleast first and second optical resonators, at least one of which istunable, each of the at least first and second optical resonators beingconnected between two of the at least first, second and third opticalcouplers, and wherein the first connection port of the first opticalcoupler receives an input optical signal, the second and thirdconnection ports of the first optical coupler are coupled to the firstof the at least first and second optical resonators, the first andsecond connection ports of the second optical coupler are coupled to thefirst of the at least first and second optical resonators, the third andfourth connection ports of the second optical coupler are coupled to thesecond of the at least first and second optical resonators, the firstand second connection ports of the third optical coupler are coupled tothe second of the at least first and second optical resonators, and thethird connection port of the third optical coupler outputs a filteredoptical signal.

In addition, there is provided in accordance with other preferredembodiments of the invention, an optical filter of the compoundresonator type, as described in the previous two paragraphs and whereinthe optical resonators consist of loops of optical transmission mediumdiffering in length from each other by predetermined amounts, or whereinthis difference in length is controlled by means of a piezoelectrictransducer operative to stabilize the length.

In accordance with still another preferred embodiment of the invention,there is provided an optical filter, of the compound resonator type, asdescribed in the previous paragraphs, and wherein at least one of theoptical couplers or one of the optical resonators is formed on anintegrated optics substrate.

In accordance with yet a further preferred embodiment of the invention,there is provided an integrated optical filter, of the compoundresonator type, consisting of at least three optical couplers, at leastone of which is formed on an integrated optics substrate, each of the atleast three optical couplers having at least three connection ports, andat least two optical resonators, at least one of which is formed on anintegrated optics substrate, each of the at least two optical resonatorsbeing connected between two of the at least three optical couplers, andwherein the first connection port of the first optical coupler receivesan input optical signal, and the last connection port of the lastoptical coupler outputs a filtered optical signal.

There is further provided in accordance with yet another preferredembodiment of the invention, an integrated optical filter, of thecompound resonator type, consisting of at least first, second and thirdoptical couplers, at least one of which is formed on an integratedoptics substrate, each of the at least first, second and third opticalcouplers having at least first, second and third connection ports, andat least first and second optical resonators, at least one of which isformed on an integrated optics substrate, each of the at least first andsecond optical resonators being connected between two of the at leastfirst, second and third optical couplers, and wherein the firstconnection port of the first optical coupler receives an input opticalsignal, the second and third connection ports of the first opticalcoupler are coupled to the first of the at least first and secondoptical resonators, the first and second connection ports of the secondoptical coupler are coupled to the first of the at least first andsecond optical resonators, the third and fourth connection ports of thesecond optical coupler are coupled to the second of the at least firstand second optical resonators, the first and second connection ports ofthe third optical coupler are coupled to the second of the at leastfirst and second optical resonators, and the third connection port ofthe third optical coupler outputs a filtered optical signal.

Additionally, there is provided in accordance with a further preferredembodiment of the invention, an integrated optical filter of thecompound resonator type wherein at least one of the optical resonatorsis tunable.

In accordance with another preferred embodiment of the invention, thereis further provided an optical filter wherein the tunable opticalresonator is tuned by altering the phase of an optical signal traversingthrough it by means of a phase modulator.

In addition, there is provided in accordance with yet another preferredembodiment of the invention, an optical filter whose filtered opticaloutput is converted to an electronic signal by means of a photodetector.

There is additionally provided in accordance with yet another preferredembodiment of the invention, an active wavelength division multiplexingsystem including an optical filter as described in this invention, thefilter being operative to select a desired wavelength of an opticalsignal.

When the tunable optical filter is implemented using bulk fiber opticalcomponents, a loop of fiber acts as the tuned resonator and a bulkelectro-optical phase modulator is inserted in the loop to provide thevariable phase delay which provides the resonator with its tunability.In order to miniaturize the filter to make it compatible with the otherintegrated opto-electronic components of an optical communicationssystem, and in order to provide the very short loop lengths needed tomeet the required specifications of the filter for dense WDM use, and inorder to reduce the manufacturing costs of such a filter, the filter canalso be implemented on a monolithic integrated optics chip, such as ofgallium arsenide, with all of the component parts defined by means ofstandard semiconductor manufacturing techniques.

The operation of the filters is based on an interferometric interactionbetween the input optical signal and its delayed produced as a result ofthe signal traversing the resonator. When the input signal and thedelayed signal are in phase at the output port of the last coupler, anoutput signal is obtained at a specific wavelength, and with theappropriate design, the device operates as an optical narrow bandpassfilter. The output spectrum characteristics can be controlled by thephase delay introduced by the phase modulator, so that the device canperform dynamic interferometric processing of the optical signal,creating a dynamic tunable filter. Since optical phase modulation can beperformed at exceedingly high rates, the result is an ultra-fast tunablewavelength selective filter. Current technology phase modulators arecapable of operation in the 10 GHz range, so that times of the order ofone nanosecond are attained. The filter thus offers an attractivesolution for the ultra-fast tuning speeds required in Tbit/secpacket-switched WDM networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings, in which:

FIG. 1 is a schematic view of a single resonator tunable delay-lineoptical filter constructed and operated according to a preferredimplementation of the present invention, showing a single opticalcoupler, a loop of optical transmission line with a phase modulatoracting as a tuned resonator, and an optional photodetector forconverting the outputted optical signal to an electronic signal ifrequired.

FIG. 2 is a schematic view of a single resonator tunable delay-lineoptical filter constructed and operated according to another preferredimplementation of the present invention, which differs from theimplementation shown in FIG. 1 in that an additional optical coupler isinserted into the resonator loop, and the filtered output optical signalextracted from this second coupler

FIG. 3 is a representation of a 5-node WDM network frequency comb, with100 GHz channel spacing, and 20 GHz channel bandwidth, and the frequencycomb of a single resonator filter for use in the at WDM system,constructed according to the present invention.

FIG. 4 is a schematic view of a compound resonator tunable delay-lineoptical filter constructed and operative according to another preferredembodiment of the present invention.

FIG. 5 shows frequency response transmission plots for a singleresonator tunable filter, constructed and operated according to apreferred embodiment of the present invention.

FIG. 6 presents theoretical frequency response transmission plots forthe compound resonator tunable filter shown schematically in FIG. 3,illustrating the advantages of the compound resonator filter over thesingle resonator filter.

FIG. 7 shows the frequency response transmission plots for threecompound resonator filters constructed with different couplers,illustrating the variation in filter finesse attainable thereby.

FIG. 8 shows a computer simulation of frequency response transmissionplots for three cases of wavelength misalignment between the transmitterlaser and a compound resonator tunable filter used in the receiver of anoptical communication link.

FIG. 9 presents plots of the BER (bit error rate) of the opticalcommunication system with the three wavelength misalignment casesdescribed in FIG. 8.

FIG. 10 shows a graph of the optical transmission as a function of thetuning of a single resonator tunable filter, constructed and operatedaccording to a preferred embodiment of the present invention, forsignals of different optical polarization.

FIGS. 11A and 11B show an additional embodiment of the presentinvention, in the form of a single resonator tunable delay line opticalfilter implemented on a monolithic integrated optics substrate ofgallium arsenide. FIG. 11A shows a schematic layout view of the circuiton the chip, whilst FIG. 11B is a cut-away cross section of the chip,showing the microelectric structure of the chip.

FIGS. 12A and 12B illustrate a more advanced embodiment of themonolithic optical filter, including three couplers and three gates, forproviding dynamic control both of the filter center frequency, and ofthe filter finesse value. FIG. 12A shows a schematic layout view of thecircuit on the chip, whilst FIG. 12B is a cut-away cross section of thechip, showing the microelectronic structure of the chip.

FIG. 13 shows an additional embodiment of the present invention, in theform of a compound resonator tunable delay line optical filterimplemented on a monolithic integrated optics substrate of galliumarsenide.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is now made to FIG. 1, which shows the construction andoperation of a single resonator tunable delay line optical filteraccording to a preferred embodiment of the present invention. Itcomprises an optical coupler 6 with four ports labeled 1 to 4, a fiberloop 8, and an optical phase modulator 10. The fiber loop 8 is connectedbetween ports 3 and 2 of the coupler 6, and thus interacts with thecoupler as a resonant element. The optical coupler 6 is designed tooptimize the filter performance in terms of transmitted power and filterfinesse.

The optical signal is inputted through port 1, and the filters signal isoutputted through port 4. If an electronic data signal output isrequired for the end use application, such as for communicationssystems, the optical output signal is converted into an electronic datasignal by means of photodetector 12. For optical signal output use, suchas in an optical spectrum analyzer, the output signal is taken directlyfrom port 4. The embodiment shown in FIG. 1 acts as a stop band filter,transmitting all wavelengths except that selected by the filter.

A further embodiment of the present invention is shown in FIG. 2, andoperates as a pass band filter, transmitting only the wavelengthselected by the filter. This embodiment differs from the embodimentshown in FIG. 1 in that the filtered output signal is extracted from theresonator by means of an additional optical coupler 14 inserted into theresonator loop 8. In this embodiment, use may be made of couplers havingonly 3 ports, 1, 2 and 3 for the input coupler 6, and 11, 12 and 13 forthe output coupler 14. The fourth port of each of these couplers isterminated internally with a built-in absorber. The output signal istaken from port 11 of the output coupler 14, either as an opticaloutput, or converted into an electronic signal by means of photodetector15.

The performance of these filters can be calculated using Fabry-Perotresonator theory, as described in a number of standard works onOpto-electronics, such as for instance, in “Guided WaveOptoelectronics”, Chapter 3, Springer Verlag, 1990. by T. Tamir. Usingthe nomenclature of the simplest embodiment shown in FIG. 1, it can beshown that the normalized light intensity l_(filter) at the output port4 is described by: $\begin{matrix}{{I_{filter} = \frac{\left( {\sqrt{R} - ^{\delta}} \right) \cdot \left( {\sqrt{R} - ^{- {\delta}}} \right)}{\left( {1 - t_{23}} \right)^{2} + {4t_{23}{\sin^{2}\left( \frac{\pi \quad {{nL}\left( {v - v_{0}} \right)}}{c} \right)}}}},} & (1)\end{matrix}$

where,

₂₃ is the complex transfer coefficient of the field amplitude from port2 to port 3 of the coupler 6,

δ is the total optical field phase delay after propagating through loop8,

R is the power reflection coefficient of the coupler 6, i.e. |t₂₃ | ²

v is the optical frequency,

n is the refractive index of the optical medium constituting the loop 8,

c is the speed of light,

L is the loop length, and

v₀ is the resonance frequency satisfying the condition: $\begin{matrix}{{\frac{2\pi \quad v_{0}{nL}}{c} = {2k\quad \pi}},} & (2)\end{matrix}$

k being any integer.

The resulting frequency characteristic of the filter is a comb of anarrow passband frequencies, one frequency for each value of k.

An important figure of merit for characterizing the wavelengthselectivity of filters is the finesse. The finesse is defined as theratio between the free spectral range (FSR), which is the frequencyrange between two resonance frequencies, and the full width half maximum(FWHM) of the filter. $\begin{matrix}{{Finesse} = {\frac{FSR}{FWHN}.}} & (3)\end{matrix}$

The free spectral range (FSR) is function of the loop length, and isgiven by: $\begin{matrix}{{FSR} = \frac{c}{nL}} & (4)\end{matrix}$

The phase modulator 10 inserted within the fiber loop is operative tocontrol the phase delay of the signal traversing the fiber loop. Thisenables external control of the phase matching condition of the fiberloop resonator. A change of the phase matching condition leads to afrequency shift of the resonance frequency v₀, proportional to the phaseshift imposed by the phase modulator. This mechanism essentially addsdynamic tuning capability to the filter. Currently available phasemodulators, such as those based on LiNbO₃ technology, are capable ofhigh-speed operation in the multi-GHz range. The filter according to thepresent invention can therefore be turned in extremely short timeperiods, of the order of a nanosecond. This is approximately threeorders of magnitude faster than other currently known filters.

The phase displacement Δδ imposed by the phase modulator causes a shiftΔv in the optical resonance frequency v₀, given by: $\begin{matrix}{{\Delta \quad v} = \frac{c\quad \Delta \quad \delta}{\pi \quad {nL}}} & (5)\end{matrix}$

so that the normalized light intensity l_(filter) at the output port 4of the phased tuned filter is described by equation (1), but with theterm v₀ replaced by v₀÷Δv, as follows: $\begin{matrix}{I_{filter} = {\frac{\left( {\sqrt{R} - ^{\delta}} \right) \cdot \left( {\sqrt{R} - ^{- {\delta}}} \right)}{\left( {1 - t_{23}} \right)^{2} + {4t_{23}\sin^{2}\left\{ {\pi \quad {{{nL}\left\lbrack {v - \left( {v_{0} + \frac{c\quad {\Delta\delta}}{\pi \quad {nL}}} \right)} \right\rbrack}/c}} \right\}}}.}} & (6)\end{matrix}$

In order to design a filter for dense WDM applications; two importantconditions must be fulfilled:

(1) the filter must reject all unselected channels within the WDM band,which is achieved by providing a sufficiently large free spectral range(FSR), and

(2) the filter must minimize crosstalk from adjacent unselectedchannels, which is achieved by constructing the filter with a highfinesse value design.

Although the single-resonator tunable filter, when constructed usingpractical lengths of optical fiber, performs well in terms of filterfinesse (typically above 1250), its FSR is not satisfactory since only20 GHz separates between two adjacent channels. A much more realisticrequirement is an FSR of the order of 100 GHz, equivalent to about 0.8nm, which is a candidate for the standard channel spacing in dense WDMsystems. The filter FSR should be slightly larger than the channelspacing:

FSR=Δ+FWHM,   (7)

where

α is the channel spacing of the WDM system, and

FWHM is the filter bandwidth.

Accordingly, all N channels adjacent to the selected one will berejected, where $\begin{matrix}{N = {{\frac{FSR}{FWHM} - 2} = {{{Finesse} - 2} \cong {{Finesse}.}}}} & (8)\end{matrix}$

In the upper section of FIG. 3 is a representation of a typical 5-nodeWDM network frequency comb, with 100 GHz channel spacing and 20 GHzchannel bandwidth, and in the lower section, the frequency comb of asingle resonator filter constructed according to the present inventionfor use in that WDM system, and having an FSR of 120 GHz typically, anda 20 GHz FWHM bandwidth.

As illustrated in FIG. 3 and shown by equation (8), such as WDM systemwould be capable of supporting 5 channels at any given time—1 selectedchannel and 4 rejected channels.

However, because of the performance limitations of the currentlyavailable DFB and DBR semiconductor lasers used in optical datacommunications systems, and particularly because of the instability ofthe central wavelength, a more conservative design is required. Equation(7) should be amended to:

FSR=Δ+k·FWHM,   (9)

where k is a factor which depends on the variance of the centralwavelength. Using this design, the number of nodes that the tunablefilter can support becomes: $\begin{matrix}{N \cong {\frac{1}{k} \cdot {Finesse}}} & (10)\end{matrix}$

However, according to equation (4), in order to achieve an FSR of theorder of 100 GHz with a single fiber loop, the required loop length isabout 2 mm. This length is totally unrealistic for practicalconstruction of single resonator filters using fiber optical loops. Twofurther embodiments of the present invention are thus proposed forproviding practical solutions to this problem.

In the next section, an embodiment of the present invention using acompound-resonator structure implemented using optical fiberconstruction is proposed. The compound resonator filter design removesthe severe loop length limitation mentioned above.

In a later section, a further embodiment of the present invention with asingle resonator structure is proposed, but the loop length problem isovercome to a large extent by implementation of the filter on amicroscopic scale on an integrated optics Gallium Arsenide substrate,using standard semiconductor manufacturing techniques

FIG. 4 shows a compound resonator tunable delay line optical filterconstructed and operated according to another preferred embodiment ofthe present invention. It comprises three optical couplers 20, 21, 22,two fiber optical loops 24, 25, denoted by the terms L1 and L2, and anoptical phase modulator 30. The optical signal is inputted through port1 of the coupler 20, and the filtered signal is outputted through port 4of coupler 22. As previously, if an electronic output signal isrequired, the optical signal is converted by means of photodetector 32.The three optical couplers need not be identical, and can be selected tooptimize the filter performance in terms of transmitted power and filterfinesse. It can be shown that the compound filter has the followingtransfer function. $\begin{matrix}{E_{out} = {E_{i\quad n} - \frac{t_{1}t_{2}t_{3}{\exp \left\lbrack {- {{\beta}\left( {L_{1} + L_{2}} \right)}} \right\rbrack}}{\begin{matrix}\left\{ {1 - {r_{1}r_{2}{\exp \left( {2i\quad \beta \quad L_{1}} \right)}} - {r_{1}r_{3}t_{2}^{2}\frac{\exp \left\lbrack {{- 2}i\quad {\beta \left( {L_{1} + L_{2}} \right)}} \right\rbrack}{1 - {r_{2}r_{3}{\exp \left( {{- 2}i\quad \beta \quad L_{2}} \right)}}}}} \right\} \\\left\lbrack {1 - {r_{2}r_{3}{\exp \left( {{- 2}i\quad \beta \quad L_{2}} \right)}}} \right\rbrack\end{matrix}}}} & (11)\end{matrix}$

where

L₁ and L₂ are the single-pass lengths of each resonator loop,

β is the propagation constant in the fiber,

t₁ and r₁ are the field amplitude transmission and reflectioncoefficients of optical coupler t, respectively, and are defined by:

t ₁ =t ₁₃ =t ₂₄ r ₁ =t ₁₄ =t ₂₃,   (12)

where t₄ is the complex transfer coefficient from port i to port j ofthe coupler.

The compound resonator filter has much more design flexibility, due tothe additional loop resonator, and the additional independenttransmission and reflection coefficients of the additional opticalcouplers. This allows the construction of filters with a wide selectionrange of FSR, FWHM, and filter finesse. In particular, it is feasibleand practical to realize a filter with FSR in the order of hundreds ofGHz, i.e., a few nm, as required for dense WDM applications in opticalcommunications.

FIG. 5 shows transmission plots for a single-resonator tunable filter,constructed and operated according to a preferred embodiment of thepresent invention, with a loop 8 of length 1 cm, and a coupler 6 withpower splitting ratio of 10/90. The curves were calculated from equation(1). The different curves, plotted for phase shifts of 0. π/4 and π/2,show the level of turnability achievable in such a filter using phasemodulation. In this example, the filter finesse exceeds 125.

FIG. 6 presents theoretical frequency response transmission plots forthe compound resonator tunable filter shown schematically in FIG. 4. Thecompound resonator configuration has major advantages over the singleresonator design. Most important is that the FSR can be extended to therange of hundreds of Ghz (a fewer nm.) and more. This is achieved usingan appropriate design of the two fiber loop resonators L1 and L2, 24 and25, with a very slight length difference between them, of the order offractions of a mm. This length difference can be accurately controlledwith a PZT (piezo-electric transducer).

A further important advantage of the compound resonator is the largesidemode rejection ratio, which is significantly increased in comparisonwith the single resonator filter. While the single resonator filterexhibits 10 dB sidelobe suppression, that of the compound resonatorfilter whose results are shown in FIG. 6 exceeds 40 dB.

Furthermore, the filter finesse can be designed more flexibly, sincethere are more independent coupler reflection and transmissionparameters to use for optimization of the desired finesse. The filter ofFIG. 6 has a finesse of over 1000. This filter is constructed with afiber loop length of 20 cm, and a loop length difference of 0.1 mm. Theinput coupler 20 has a power splitting ratio of 2/98, the intermediatecoupler 21, a ratio of 1/99, and the output coupler 22, a ratio of10/99.

FIG. 7 shows the frequency response transmission plots for threecompound resonator filters constructed using couplers with differentpower splitting ratios, illustrating the variation in filter finesseattainable thereby. Response curve 1 is obtained from a compoundresonator filter with coupler splitting ratios of 20/80, 8/92 and 10/90for the input, intermediate and output couplers respectively, and theresulting filter has a finesse of about 25. Response curve II hascouplers of splitting ratios 10/90, 1/99 and 10/90 respectively,resulting in a finesse of the order of 250. Response curve III hascouplers of splitting ratios 1/99, 0.1/99.9 and 10/90 respectively, andthe filter a finesse of 1000.

The ability to select the filter finesse is an important feature of thecompound resonator filter, since different optical communicationssystems require different wavelength selective curves to optimize systemperformance. One important consideration in filter design considerationis the wavelength stability of the transmitter laser used in the system.If the wavelength stability of the laser is such that there could arisesignificant wavelength misalignment between the laser frequency and thefilter mid-band frequency, then a filter with lower finesse is requiredto compensate for this misalignment.

FIG. 8 shows a computer simulation of frequency response transmissionplots for three cases of wavelength misalignment between the transmitterlaser and the filter at the receiver of an IM/DD(intensity/modulation/direct-detection) optical communication link. Thelaser spectrum chosen was one with a Lorenzian lineshape and a bandwidthof 1 GHz, which would represent the modulation of a datastream runningat 1 Gbit/sec. The spectral response curves of the filter are generatedfrom equation (11), while the filter finesse is 25. The three differentcurves represent the following cases: a perfectly aligned case (curve1), a 15 GHz wavelength misalignment (curve II) and a 30 GHz wavelengthmisalignment (curve III). As is evident, when the wavelengthmisalignment is large (15 or 30 GHz), a filter of low finesse isessential to compensate for the misalignment.

FIG. 9 presents plots of the BER (bit error rate) of an IM/DD opticalcommunication system with the three wavelength misalignment casesdescribed in FIG. 8. The filter itself uses couplers with powersplitting ratios of 50/50, 1/99 and 10/90, and has a finesse of 25. Ituses a PIN photodiode for detection, and the whole system has an NEP of30 pW/✓Hz, and a responsitivity of 4. The transmission bit rate is 1.25Gbit/sec. It is observed that for a system with a perfectly alignedlaser, as shown by curve 1, the receiver sensitivity, defined at a BERof 10⁻⁹, is −18.5 dBm. The same system using a laser with wavelengthmisalignment of 15 Ghz, as shown in curve 11, suffers a power penalty ofabout 2 dBm, while a 30 GHz misalignment, as shown in curve III,produces a loss of about 7.5 dBm compared with the perfectly alignedcase. These noise figures are obtained with a filter of relatively lowfinesse, 25, specifically selected to compensate for the lasermisalignment. If a narrower band filter with high finesse were used, thepower penalty with wavelength misalignment becomes much higher.

On the other hand, the use of a filter with a very low finesse increasesthe power penalty as a result of crosstalk between adjacent channels.Therefore, an optimization procedure must be followed to minimize thepower penalties derived from both wavelength misalignment and channelcrosstalk.

Because of birefringence in the active optical medium in the phasemodulator, transmission of the TE and TM polarization components of theoutput of the filter will change with the wavelength to which the filteris tuned. This effect allows yet a further embodiment of the presentinvention, whereby the filter acts as a polarization selector forseparating TE and TM components of a mixed polarization signal. In thisembodiment, an additional polarizer must be used in the output or inputline to remove the polarization component not required. FIG. 10 is agraph of the optical transmission as a function of the tuning of thecompound resonator tunable filter of FIG. 6., which illustrates how afilter constructed and operative according to a preferred embodiment ofthe present invention, can perform polarization selection.

All of the above described implementations of the present invention,using optical fibers, are comparatively expensive to manufacture, sinceeach component part has to be assembled in the filter and fine tunedindividually. Furthermore, the use of the bulk fiber configurationresults in a bulky component package.

Both of these disadvantage can be overcome by means of a furtherembodiment of the present invention, whereby the filter is implementedby monolithic integration on a single opto-electronic chip, as used inintegrated optics technology. The main advantage of this integrated chipimplementation is commercial, since production using standardsemiconductor industry technology enables the filters to be manufacturedto lower cost, more reproducibly and with higher reliability. Such massproduction is essential to allow the proliferation of WDM-based opticaldata communication systems.

However, besides the commercial advantages, the integrated opticsmonolithic implementation also has a number of technological advantages.Firstly, the optical loop resonator length can be made significantlysmaller, down to the order of a millimeter or less. As a result, theresonance build-up time is considerably shorter, thus increasing thefilter tuning speed. More important, the filter free spectral range(FSR) can be increased by an order of magnitude, up to the order of ahundred GHz. This sufficient for current WDM systems using a smallnumber of laser sources, with 10 GHz bandwidth typically. In order toachieve a monolithic filter with an FSR of 5 Thz as demanded by therequirements of the currently proposed WDM networks, which will berequired to cover the whole EDFA (Ebrium Doped Fiber-Optical Amplifiers)bandwidth, a more advanced configuration must be used, such as thecompound resonator filter described previously.

If the monolithic embodiment is constructed without incorporating aphase modulation element, a fixed wavelength monolithic filter isobtained, which can be constructed with selected center wavelength andfinesse according to the parameters and dimensions chosen. Because ofits small size and superior properties, such filters are useful forstatic switched WDM optical communication system applications.

In addition, the losses within the device can be decreased. The fiberloop configuration includes connections to an external phase modulator,which introduces losses of an additional pair of connectors (about 0.2dB each). The integrated optics configuration incorporates the phasemodulator on the same chip. Therefore, losses are reduced because of thereduction in the use of one set of connectors.

Furthermore, temperature dependence can be overcome quite simply, sincethe whole device is integrated on a very small chip, which can be keptat fixed temperature by means of simple and inexpensive control methods.

Finally, the monolithic design leads to a very small device of sizesimilar to that of semiconductor lasers and suitable for integrationwith other OEIC (Optoelectronic integrated circuit) components intocomplete integrated optics communications systems.

FIGS. 11A and 11B show a schematic view of an additional embodiment ofthe present invention, in the form of a single resonator tunable delayline optical filter implemented on a monolithic integrated opticssubstrate of gallium arsenide. Any other suitable integrated opticssubstrate material could also be used. FIG. 11A is a plan view of thechip layout, while FIG. 11B is a cut-away cross section of the chipshowing the microelectronic structure of the chip. The filter ismanufactured on an MBE (molecular beam epitaxy) grown n-type GaAs wafer40. Using standard photolithographic techniques, a mesa is defined andetched by RIE (reactive ion etching). The mesa defines the waveguide 42,and the resonator loop 43 of the optical structure of the filter. Thesewaveguides are defined using one of the standard cladding/core/claddingwaveguiding structures used in integrated optics GaAs technology. Ametal gate 44 over part of the resonator loop, and its associated bondpad 46, are defined by a further process. The device is covered with aninsulating layer 41 such as polyimide to passivate the device, and toinsulate the gate 46 from the wafer. A via 45 in the polyimide layerfacilitates the gate-to-resonator loop contact area. An Ohmic contact isevaporated onto the back of the wafer, and is allowed in Finally, thesubstrate is cleaved to create cleaved edge factets 47 for interface tothe input and output optical signals

The gate 44 located above a section of the resonator loop together withthe section of the loop itself, act as a phase modulation element. Thesignal applied to the gate creates an electric field across that sectionof the resonator loop, causing a change in the waveguide refractiveindex by means of a physical effect, such as the linear electro-opticeffect. The change in refractive index introduces a phase change in theoptical signal propagating in the resonator loop, analogous to thatintroduced by the Lithium Niobate phase modulator 10 of the fiberoptical implementation shown in FIG. 1. If this gate is omitted, astatic monolithic filter is obtained. The coupling between thethrough-waveguide 42 and the resonator loop 43 takes place across thegap 48, whose width and length are calculated to provide the correctpower splitting ratio for the designed filter operation.

FIGS. 12A and 12B illustrate a schematic view of a more advancedembodiment of the monolithic optical filter. FIG. 12A is a plan view ofthe chip layout, while FIG. 12B is a cut-away cross section of the chipshowing the microelectronic structure of the chip. This constructioncomprises a GaAs wafer 52 on which are defined a first waveguide 53, anda second waveguide 63, with a resonator loop 56 disposed between them. Afirst metal gate 59 is located above a section of the resonator loop,and two further gates 54 and 55 are located between each waveguide andthe resonator loop. Before deposition of the metallic gates, the wholeGaAs structure is covered with an insulating layer 58 such as polyimide,containing vias 57 to provide contact between these three gates and theunderlying GaAs layer. The manufacturing technique is similar to that ofthe simpler embodiment filter shown in FIGS. 11A and 11B.

This advanced embodiment acts as a bandpass filter, by selecting, fromthe range of wavelengths inputted to the first waveguide 53, thespecific wavelength to be switched to the second waveguide 63, by meansof the resonance loop 56. The first gate 59 together with the section ofresonator loop under it operates as a phase modulator for tuning thefilter passband. As in the embodiment of FIGS. 11A and 11B, if thisembodiment is constructed without the phase modulator gate 59, a staticfilter is obtained.

The two other gates 54 and 55 are operative to change the couplingbetween the first waveguide and the resonator loop, and between theresonator loop and the second waveguide as a function of the voltageapplied to them. In this way, the filter finesse can be changeddynamically by means of the control signals applied to gates 54 and 55.

FIG. 13 shows a schematic layout view of a compound resonator monolithictunable filter constructed and operated according to another preferredembodiment of the present invention, analogous to the fiber opticalimplementation described in FIG. 4. Use of the compound resonatormonolithic embodiment enables filters with an FSR of up to 5 Thz to beconstructed, for use in the next generation WDM system technology.

The embodiment shown in FIG. 13 is constructed on a substrate 52 ofGallium Arsenide, and has a passivating layer 58 of polyimide, as in thepreviously described monolithic embodiments. It has two resonator loops56, 70, and a variable phase modulator gate 59 acting on one of theloops 56. The phase change is varied by means of a voltage applied topad 60. This embodiment of the filter can also be constructed in a fixedwavelength form, in which case the gate 59 and pad 60 are notfabricated. The coupling between the two resonators can be varied bymeans of gate 55. Gate 54 varies the coupling between the input line 53and the first resonator 56, and gate 74, the coupling between the secondresonator 70 and the output port 76. In this way, the filter finesse canbe changed dynamically by means of control signals applied to gates 54,55 and 74. The optical signal is inputted through line 53, and thefiltered signal outputted through line 63 to the output port 76.

Some of the component parts of the monolithic implementations describedin FIGS. 10, 11 and 12 may be implemented as discrete non-integratedcomponents, if such hybrid construction is necessary, convenient oreconomical for the specific application required.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

We claim:
 1. A method for manufacturing an optical filter comprising:forming at least one multiport optical coupler on a gallium arsenidesubstrate; and forming at least one electrically tunable opticalresonator on said gallium arsenide substrate, wherein one connectionport of said at least one multiport optical coupler receives an inputoptical signal, and another connection port of said at least onemultiport optical coupler outputs a filtered optical signal, and said atleast one electrically tunable optical resonator is connected to atleast one of said at least one multiport optical coupler.
 2. A methodaccording to claim 1 and wherein: said at least one multiport opticalcoupler has at least first, second, third and fourth connection ports,said first connection port receiving an optical signal, said secondconnection port outputting a filtered optical signal; and said at leastone tunable optical resonator is connected across said third and fourthconnection ports.
 3. A method according to claim 1 and wherein: saidforming at least one multiport optical coupler comprises forming atleast first and second optical couplers, each having at least first,second and third connection ports; said first connection port of saidfirst optical coupler receiving an optical signal, said second and thirdconnection ports of said first optical coupler are coupled to said atleast one tunable optical resonator, said first and second connectionports of said second optical coupler are coupled to said at least onetunable optical resonator, and said third connection port of said secondoptical coupler outputting a filtered optical signal.
 4. A methodaccording to claim 1 and wherein said at least one tunable opticalresonator is operative to select an optical signal with a specificwavelength.
 5. A method according to claim 1 and wherein said at leastone tunable optical resonator is operative to enable the polarization ofsaid filtered optical output signal to be selected.
 6. A methodaccording to claim 1 and wherein said tunable optical resonator is tunedby altering the phase of an optical signal traversing through it bymeans of a phase modulator.
 7. A method according to claim 1 and furthercomprising forming a photodetector on said gallium arsenide substrate,wherein said photodetector converts filtered optical output to anelectronic signal.
 8. A method according to claim 1 and comprising:defining a first waveguide and a second waveguide having a resonatorloop disposed therebetween on a gallium arsenide wafer; locating a firstgate above a second gate of the resonator loop; and locating said secondgate and a third gate between said resonator, and said first waveguideand said second waveguide, respectively, said first gate and saidresonant loop operating as a phase modulator for tuning a passband ofsaid filter, said second and third gates being operative to change thecoupling between the first waveguide and the resonator loop and betweenthe resonator loop and the second waveguide as a function of voltageapplied thereto, thereby enabling filter finesse to be changeddynamically by control signals applied to said second and third gates.9. A method for manufacturing a variable finesse optical filtercomprising: forming an optical element having variable finesse on agallium arsenide substrate, said optical element receiving an opticalsignal and providing a filtered output; and forming an electricallytunable finesse controller on said gallium arsenide substrate, saidelectrically tunable finesse controller operative to select the finesseof and optical element.
 10. A method for manufacturing a variablefinesse optical filter according to claim 9 and wherein said forming anoptical element with variable finesse comprises forming an opticalcoupler with variable power splitting ratio between its connectionports.
 11. A method for manufacturing a variable finesse optical filteraccording to claim 9 and comprising: forming at least one multiportoptical coupler with variable power splitting ratio, one connection portof said at least one multiport optical coupler receiving an inputoptical signal, and another connection port of said at least onemultiport optical coupler outputting a filtered optical signal; andforming at least one tunable optical resonator; and connecting said atleast one tunable optical resonator to at least one of said at least onemultiport optical coupler.
 12. A method for manufacturing a variablefinesse optical filter according to claim 11 and comprising: forming atleast first and second optical couplers, at least one of which hasvariable power splitting ratio, and each having at least first, secondand third connection ports; coupling said second and third connectionports of said first optical coupler to said at least one tunable opticalresonator; and coupling said first and second connection ports of saidsecond optical coupler to said at least one tunable optical resonator,said first connection port of said first optical coupler receiving anoptical signal, and said third connection port of said second opticalcoupler outputting a filtered optical signal.
 13. A method formanufacturing a variable finesse optical filter according to claim 11and wherein said at least one tunable optical resonator is operative toselect an optical signal with a specific wavelength thereby providingtunability to both the wavelength and finesse of said optical filter.14. A method for manufacturing a variable finesse optical filteraccording to claim 11 and wherein said at least one tunable opticalresonator is operative to enable the polarization of said filteredoptical output signal to be selected.
 15. A method for manufacturing avariable finesse optical filter according to claim 11 and wherein saidtunable optical resonator is tuned by altering the phase of an opticalsignal traversing through it by means of a phase modulator.
 16. A methodfor manufacturing a variable finesse optical filter according to claim 9and comprising: forming at least one multiport optical coupler withvariable power splitting ratio, said coupler having at least first,second, third and fourth connection ports, said first connection portreceiving an optical signal, said second connection port outputting afiltered optical signal; forming at least one tunable optical resonator;and connecting said at least one tunable optical resonator across andthird said fourth connection ports.
 17. A method for manufacturing avariable finesse optical filter according to claim 9 and comprisingforming a photodetector on said gallium arsenide substrate, wherein saidphotodetector converts filtered optical output to an electronic signal.18. A method for manufacturing a variable finesse optical filteraccording to claim 9 and comprising: defining, on a gallium arsenidewafer, a first waveguide and a second waveguide having a resonator loopdisposed therebetween; and locating a first gate above a second gate ofthe resonator loop; and locating said second gate and a third gatebetween said resonator, and said first waveguide and said secondwaveguide, respectively, said first gate and said resonant loopoperating as a phase modulator for tuning a passband of said filter,said second and third gates being operative to change the couplingbetween the first waveguide and the resonator loop and between theresonator loop and the second waveguide as a function of voltage appliedthereto, thereby enabling filter finesse to be changed dynamically bycontrol signals applied to said second and third gates.
 19. A method formanufacturing an integrated optical filter comprising: forming at leastone multiport optical coupler having at least first, second, third andfourth connection ports, said first connection port receiving an opticalsignal, said second connection port outputting a filtered opticalsignal; forming at least one electrically tunable optical resonatorconnected to at least one of said multiport optical couplers; andconnecting said at least one optical resonator across said third andfourth connection ports, and wherein at least one of said one multiportoptical coupler and said at least one optical resonator are formed on agallium arsenide substrate.
 20. A method for manufacturing an integratedoptical filter comprising: forming at least one multiport opticalcoupler, one connection port of said at least one multiport opticalcoupler receiving an input optical signal, and another connection portof said at least one multiport optical coupler outputting a filteredoptical signal; and forming at least one electrically tunable opticalresonator connected to at least one of said multiport optical couplers;and wherein at least one of said one multiport optical coupler and saidat least one optical resonator are formed on gallium arsenide substrate.21. A method for manufacturing an integrated optical filter to claim 20and wherein at least one of said at least one multiport optical couplerand said at least one optical resonator includes a discretenon-integrated optical component.
 22. A method for manufacturing anoptical filter according to claim 20 and wherein said tunable opticalresonator is tuned by altering the phase of an optical signaltraversing, through it by means of a phase modulator.
 23. A method formanufacturing an integrated optical filter according to claim 20 andcomprising forming a photodetector on said gallium arsenide substrate,wherein said photodetector converts filtered optical output to anelectronic signal.
 24. A method for manufacturing an optical filteraccording to claim 20 and comprising: defining on a gallium arsenidewafer, a first waveguide and a second waveguide having a resonator loopdisposed therebetween; and locating a first gate above a second gate ofthe resonator loop; and locating said second gate and a third gatebetween said resonator, and said first waveguide and said secondwaveguide, respectively, said first gate and said resonant loopoperating as a phase modulator for tuning a passband of said filter,said second and third gates being operative to change the couplingbetween the first waveguide and the resonator loop and between theresonator loop and the second waveguide as a function of voltage appliedthereto, thereby enabling filter finesse to be changed dynamically bycontrol signals applied to said second and third gates.
 25. A method formanufacturing an integrated optical filter comprising: forming at leastfirst and second optical couplers, each having at least first, secondand third connection ports; forming at least one electrically tunableoptical resonator; and coupling said second and third connection portsof said first optical coupler to said at least one optical resonator;and coupling said first and second connection ports of said secondoptical coupler to said at least one optical resonator, and wherein atleast one of, said optical couplers and said at least one opticalresonator, are formed on a gallium arsenide substrate, said firstconnection port of said first optical coupler receives an opticalsignal, and said third connection port of said second optical coupleroutputs a filtered optical signal.
 26. A method for manufacturing anoptical filter comprising: forming at least first, second and thirdoptical couplers on a gallium arsenide substrate, each having at leastfirst, second and third connection ports; forming at least first andsecond optical resonators on said gallium arsenide substrate, at leastone of which is tunable; coupling said second and third connection portsof said first optical coupler to the first of said at least first andsecond optical resonators; coupling said first and second connectionports of said second optical coupler to the first of said at least firstand second optical resonators; coupling said third connection port and afourth connection port of said second optical coupler to the second ofsaid at least first and second optical resonators; and coupling saidfirst and second connection ports of said third optical coupler to thesecond of said at least first and second optical resonators, and whereinsaid first connection port of said first optical coupler receives aninput optical signal, said third connection port of said third opticalcoupler outputs a filtered optical signal.
 27. A method formanufacturing an optical filter comprising: forming at least threeoptical couplers on a gallium arsenide substrate, each having at leastthree connection ports; and forming at least two optical resonators onsaid gallium arsenide substrate, at least one of which is electricallytunable, connecting each of said at least two optical resonators betweentwo of said at least three optical couplers; and wherein a firstconnection port of a first one of said at least three optical couplersreceives an input optical signal, and a last connection port of a lastone of said at least three optical couplers outputs a filtered opticalsignal.
 28. A method for manufacturing an optical filter according toclaim 27 and wherein said optical resonators comprise loops of opticaltransmission medium differing in length from each other by predeterminedamounts.
 29. A method for manufacturing an optical filter according toclaim 28 and wherein said difference in length of said loops of opticaltransmission medium is controlled by means of a piezoelectric transduceroperative to stabilize the resonator length.
 30. A method formanufacturing an optical fiber according to claim 27 and wherein said atleast one optical resonator is tuned by altering the phase of an opticalsignal traversing through it by means of a phase modulator.
 31. A methodfor manufacturing an optical filter according to claim 27 and comprisingforming a photodetector on said gallium arsenide substrate, wherein saidphotodetector converts filtered optical output to an electronic signal.32. A method for manufacturing an optical filter according to claim 27and comprising: defining, on a gallium arsenide wafer, a first waveguideand a second waveguide having a resonator loop disposed therebetween;and locating a first gate above a second gate of the resonator loop; andlocating said second gate and a third gate between said resonator, andsaid first waveguide and said second waveguide, respectively, said firstgate and said resonant loop operating as a phase modulator for tuning apassband of said filter, said second and third gates being operative tochange the coupling between the first waveguide and the resonator loopand between the resonator loop and the second waveguide as a function ofvoltage applied thereto, thereby enabling filter finesse to be changeddynamically by control signals applied to said second and third gates.33. A method for manufacturing an integrated optical filter comprising:forming at least three optical couplers, at least one of which is formedon a gallium arsenide substrate, each of said at least three opticalcouplers having at least three connection ports; forming at least twooptical resonators, at least one of which is formed on a galliumarsenide substrate, at least one of which is electrically tunable; andconnecting each of said at least two optical resonators between two ofsaid at least three optical couplers; and wherein a first connectionport of a first one of said at least three optical couplers receives aninput optical signal, and a last connection port of a last one of saidat least three optical couplers outputs a filtered optical signal.
 34. Amethod for manufacturing an integrated optical filter comprising: atleast first, second and third optical couplers, at least one of which isformed on an integrated optics substrate, each of said at least first,second and third optical couplers having at least first, second andthird connection ports; and at least first and second opticalresonators, at least one of which is formed on an integrated opticssubstrate, each of said at least first and second optical resonatorsbeing connected between two of said at least first, second and thirdoptical couplers; and wherein: said first connection port of said firstoptical coupler receives an input optical signal, said second and thirdconnection ports of said first optical coupler are coupled to first ofsaid at least first and second optical resonators, said first and secondconnection ports of said second optical coupler are coupled to first ofsaid at least first and second optical resonators, said third connectionport and a fourth connection port of said second optical coupler arecoupled to second of said at least first and second optical resonators,said first and second connection ports of said third optical coupler arecoupled to second of said at least first and second optical resonators,and said third connection port of said third optical coupler outputs afiltered optical signal.